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Sun Nyunt WaiMolecular mechanisms of bacterial pathogenesis

PI: Sun Nyunt Wai MD, PhD
Professor of Medical Microbial Pathogenesis
Department of Molecular Biology
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Our main goal is to understand the mechanisms used by pathogenic bacteria in their interaction within a host and for their survival in different environments contributing to the intestinal and extra-intestinal pathogenesis, environmental survival and emergence of new pathotypes. We study the Vibrio cholerae because it is a major pathogen responsible for the life-threatening acute diarrhoea, cholera, which is a worldwide problem. V. cholerae is not only an important bacterial pathogen but it also serves as a model system for studies of many important aspects of medical microbial pathogenesis, e.g. how bacteria regulate and express virulence factors, cause damage in the host, and adapt to different environmental conditions are thereby important issues. Using new approaches we have shown how the bacteria may modulate expression of virulence and how the bacteria transport virulence factors via membrane vesicles. We have identified factors and possible mechanisms contributing to both environmental persistence and to their balanced interaction with the human host.

Current projects:
  1. Role of bacterial outer membrane vesicles (OMVs) in host interactions and transport of virulence factors
  2. Small regulatory RNA and virulence gene regulation in V. cholerae
  3. Type VI secretion system of serotype O1 V. cholerae
Micrograph V cholerae
Electron micrograph of V. cholerae (left), Electron micrograph of V. cholerae with OMVs

Project 1: Role of OMVs and virulence factors transport
Gram-negative bacteria release OMVs during their normal growth. OMVs are formed by protrusions from the outer membrane and contain certain periplasmic proteins, lipopolysaccharides (LPS), outer membrane proteins (OMPs), surface phospholipids, and peptidoglycans. In our lab we investigate the composition and biological significance of OMVs derived from clinical and environmental strains of V. cholerae. We observed that these OMVs can elicit immune responses in human host cells. Particularly, we observed a plausible role of OMV-associated peptidoglycans in inducing the NOD1 and NOD2 mediated inflammatory response in mammalian cells. Currently, we are investigating different steps involved in the uptake of OMVs by the host cells and the nature of biological role played by different OMV associated virulence factors.

Project 2: Small regulatory RNA and virulence gene regulation
RNA regulators in bacteria are a heterogeneous group of molecules that act by various mechanisms to modulate a wide range of physiological responses. Small regulatory RNA (sRNA) is one subset of such RNA regulators. The majority of these sRNAs regulate responses to changes in environmental conditions.
Our group discovered a new sRNA, named VrrA, in V. cholerae. VrrA regulates several important proteins of the organism. The vrrA mutant showed a fivefold increased ability to colonize the intestine of infant mice as compared with the wild type.
Our data suggest that VrrA is an important regulator for the bacterium to adapt to different environments, both outside and inside of the host. We are currently identifying additional targets of VrrA in order to uncover the full regulatory network.

Project 3: Type VI secretion system of serotype O1 V. cholerae
V. cholerae has evolved several secretion systems to export toxins, enzymes, and other proteins necessary for the bacterial growth and survival in different environments and for bacteria-host interactions. The recent discoveries include the type VI secretion system (T6SS) that appears to be common in many different Gram-negative bacteria.
We have previously shown that the expression of the effecter protein Hcp is regulated in a population-dependent manner (quorum-sensing system) in the V. cholerae O1 strains. Currently, we found that high osmolarity and low temperature conditions could induce the secretion of Hcp and that this system plays a role in inter-bacterial virulence of V. cholerae O1 strains.

Selected publications
Wai SN, Lindmark B, Söderblom T, Takade A, Westermark M, Oscarsson J, Jass J, Richter-Dahlfors A, Mizunoe Y, Uhlin BE. 2003. Vesicle-mediated export and assembly of pore-forming oligomers of the enterobacterial ClyA cytotoxin. Cell. 2003 115(1):25-35.

Song T, Mika F, Lindmark B, Liu Z, Schild S, Bishop A, Zhu J, Camilli A, Johansson J, Vogel J, Wai SN. 2008. A new Vibrio cholerae sRNA modulates colonization and affects release of outer membrane vesicles. Mol Microbiol 70:100–111.

Song T, Sabharwal D, Wai SN. 2010. VrrA mediates Hfq-dependent regulation of OmpT synthesis in Vibrio cholerae. J Mol Biol. 400(4):682-8.

Ishikawa T, Rompikuntal PK, Lindmark B, Milton DL, Wai SN. 2009. Quorum sensing regulation of the two hcp alleles in Vibrio cholerae O1 strains. PLoS One 24;4(8):e6734.

Ishikawa T, Sabharwal D, Bröms J, Milton DL, Sjöstedt A, Uhlin BE, Wai SN. 2012.
Pathoadaptive conditional regulation of the type VI secretion system in Vibrio cholerae O1 strains. Infect Immun. 80(2):575-84.

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